The Swan-Ganz catheter provides useful hemodynamic data that can guide therapy of critically ill patients. The use of this catheter has extended to many areas other than intensive care unit, coronary care unit, the cardiovascular surgical suite, the recovery room, the neurosurgical and obstetric areas, the cardiac catheterization and pulmonary function labs (Swan-Ganz, 1975). Nurses in many areas of practice must be familiar with this method of monitoring. The nurse is responsible for calibrating the equipment measuring and recording pressures as indicated. She is also responsible for using pressure measurements as vital data in assessing the patient's condition and in evaluating response to therapy. It is common practice to position the patient supine and flat, and to disconnect the patient who is on intermittent positive pressure ventilation from the ventilator for pulmonary artery and pulmonary capillary wedge pressure measurements. These practices are thought to give valid measurements. Measurements are recorded hourly or more often depending upon the condition of the patient. This may require waking the acutely ill patient to reposition him flat, and also, there are patients who should not be placed flat in bed, e.g., patients in respiratory distress. Disconnecting the patient from the ventilator may also disturb a patient, increase his anxiety and expose him to potential hypoxia during the procedure. The degree of distress imposed is dependent upon the speed and skill of the person measuring these pressures. It is unknown whether accurate PA and PCW pressures can be obtained if the patient is not positioned flat and not disconnected from the ventilator. Recent preliminary work questions the necessity for these practices during pressure measurements. Further data, collected under controlled conditions with use of specialized equipment, are needed to answer these questions. If no difference is found, then accurate pressure measurements could be obtained without disturbing the acutely ill patient, hourly or more often; by positioning him flat and by disconnecting him from the ventilator. Perphaps patients are being disturbed frequently when it is not necessary to do so.